This invention relates to a gear speed-changing apparatus for an automatic transmission, which is constructed to include an input portion, three sets of planetary gears, three clutches, two brakes and an output portion and which is enabled to achieve at least six forward speeds and one reverse speed by engaging/releasing the three clutches and the two brakes properly as speed-changing elements.
A gear speed-changing apparatus for an automatic transmission, which is constructed to include an input shaft, one set of single-pinion type planetary gear, a Simpson type planetary gear train having two sets of single-pinion type planetary gearsets combined with each other, three clutches, two brakes and an output shaft and which is enabled to achieve six forward speeds and one reverse speed by engaging/releasing the three clutches and the two brakes properly as speed-changing elements, has been proposed in the prior art in FIG. 7 of JP-A-4-219553.
This gear speed-changing apparatus constructed of one single-pinion type planetary gear and a Simpson type planetary gear train has the features, as enumerated in the following.
(1) An advantage is taken at the strength because the flow of the torque transmission at the 1st speed for the highest torque of the Simpson type planetary gear train is borne through all members.
(2) An advantage is taken at the gear strength, the gear lifetime, the carrier rigidity and so on, because the Simpson type planetary gear train employs the ring gear input so that the tangential force is reduced to about one half of that of the sun gear input.
(3) For the gear range of an overdrive, there is needed a carrier input to the Simpson type planetary gear train. If the input shaft and the output shaft are coaxially arranged, the input route to the carrier is not satisfied by the single-pinion type planetary gear in which the number of rotary members is limited to three. In order to secure the input route to the carrier, therefore, the input shaft and the output shaft have to be positioned in parallel on the different axis thereby to enlarge the size of the automatic transmission.
In order to solve the aforementioned problem (3), therefore, a gear speed-changing apparatus using a Ravigneaux type composite planetary gear train (i.e., the composite planetary gear train having sun gears meshing individually with double pinions) in place of the Simpson type planetary gear train has been proposed in FIG. 13, FIG. 14 and FIG. 15 of JP-A-4-219553.
However, the gear speed-changing apparatus adopting that Ravigneaux type composite planetary gear train has the problems, as enumerated in the following.
(5) The strength is at a disadvantage because the maximum torque (for a 1st speed) of the gear train is borne by the single-pinion type planetary gear on one side of the Ravigneaux type composite planetary gear train.
(6) Another disadvantage resides in the gear strength, the gear lifetime or the carrier rigidity, because the torque augmented by one single-pinion type planetary gear acting as a speed-reducing device is inputted from the sun gear of the Ravigneaux type composite planetary gear train so that the tangential force becomes higher than that of the ring gear input.
(7) The Ravigneaux type composite planetary gear train has to be enlarged to enlarge the automatic transmission, because there are demanded both the securement of the strength (e.g., the gear strength or the gear lifetime) at the 1st speed and the improvement in the carrier rigidity of the Ravigneaux type composite planetary gear train.
(8) Depending upon the gear range, a torque circulation occurs in the Ravigneaux type composite planetary gear train so that the transmission efficiency drops to deteriorate the fuel economy at the gear range where the torque circulation occurs.
In short, the gear speed-changing apparatus, in which one set of single-pinion type planetary gear is combined with the Ravigneaux type composite planetary gear train, cannot be avoided in the size enlargement of the automatic transmission, because both the aforementioned advantages (1) and (2) of the gear speed-changing apparatus using the single-pinion type planetary gear and the Simpson type planetary gear train are offset and because the Ravigneaux type composite planetary gear train is enlarged for the aforementioned reason (3).
The present invention has been conceived noting the above-specified problems and has an object to provide a gear speed-changing apparatus for an automatic transmission, which can make the degree of freedom for selecting a gear ratio higher than that of the case using the Ravigneaux type composite planetary gear train, while achieving a strength advantage of the gear train, an advantage of the gear strength, the gear lifetime and so on, an improvement in the fuel economy, the coaxial positioning of the input portion and the output portion and the size reduction of the automatic transmission altogether. Another object is to provide a concrete layout of an automatic transmission for solving the above-specified problems.
According to the present invention, more specifically, in a gear speed-changing apparatus for an automatic transmission, which has speed change control means using not the Ravigneaux type composite planetary gear train but basically a gear train having two sets of combined single-pinion type planetary gears as a gear train to be combined with one set of planetary gear, thereby to establish at least six forward speeds and one reverse speed by engaging/releasing three clutches and two brakes properly,
one of the aforementioned three planetary gearsets is a speed-reducing device for reducing the speed of the input rotation always, and
one of the remaining two planetary gearsets is a double sun gear type planetary gearset including: two sun gears; a pinion meshing individually with said two sun gears; a carrier having a center member positioned between the aforementioned two sun gears for inputting or outputting a rotation; and one ring gear meshing with the aforementioned pinion.
This double sun gear type planetary gear is similar in the basic gear performance to a single-pinion type planetary gear, but is characterized in that the member number is so larger than that of the single-pinion type planetary gear having three members as (two members from the sun gear)+(one member from the ring gear)+(two axial and radial members from the carrier)=five members.
The gear train having a single-pinion type planetary gear and a double sun gear type planetary gear combined therein will be named the xe2x80x9cIshimaru type planetary gear trainxe2x80x9d by quoting the name of the inventor so that it may be discriminated from the xe2x80x9cRavigneaux type composite planetary gear trainxe2x80x9d or the xe2x80x9cSimpson type planetary gear trainxe2x80x9d,
Thus, the construction is made by combining one set of planetary gear and the Ishimaru type planetary gear train having a fundamental performance similar to that of the Simpson type planetary gear train. It is, therefore, possible to achieve both the strength advantage of the planetary gear because of the possibility of the ring gear input and the advantage of the gear strength or the gear lifetime of the planetary gear because the torque flow at the 1st speed can be borne through all members.
The construction is also made such that the Ishimaru type planetary gear train is used as the remaining two planetary gearsets but not the Ravigneaux type composite planetary gear train. Therefore, the improvement in the fuel economy can be achieved by the high transmission efficiency having no torque circulation.
Of the remaining two sets of planetary gears (or Ishimaru type planetary gear train), moreover, one planetary gearset is exemplified by the double sun gear type planetary gear having the center member positioned between the two sun gears. Therefore, an input route to the carrier can be formed to accomplish such coaxial positioning the input portion and the output portion.
With the construction not using the Ravigneaux type composite planetary gear train but positioning the input portion and the output portion coaxially, moreover, the gear speed-changing apparatus can be made compact to achieve the size reduction of the automatic transmission.
Still moreover, the Ravigneaux type composite planetary gear train is regulated by that the ring gear tooth number is constant when the gear ratio (=sun gear tooth number/ring gear tooth number) is to be set. Considering the conditions of a generally applicable gear ratio range and the smaller step ratio for the higher gear ranges, therefore, the Ishimaru type planetary gear train capable of setting the gear ratios of two planetary gearsets independently of each other can have a wider applicable gear ratio range and a higher degree of freedom for selecting the gear ratio than those of the Ravigneaux type composite planetary gear train.
Moreover, an intermediate wall, as disposed in the case and defining the case inside into one side and the other side, is constructed to form two brake cylinder chambers and to support the clutch drum of one of the three clutches. Therefore, the clutches and the brakes can be efficiently arranged to make the entire length of the transmission compact.
By positioning the clutches and the brakes altogether near the intermediate wall, moreover, the handling of the oil passages can be facilitated to make the transmission compact.
Still moreover, the intermediate wall can be sub-assembled with the brakes and the clutches, and the intermediate wall sub-assembly can be made on a line different from the main assembly line so that the transmission can be easily manufactured.